Few monomers are commercially produced in the world on so large a scale as vinyl chloride monomer ("VCM"). The U.S. production alone of VCM was about 7.7 billion pounds in the year 1984, most of which was used in the production of poly(vinyl chloride), the remainder was used for the production of copolymers of VCM with vinylidene chloride, graft copolymers of vinyl chloride on methylmethacrylate, polybutadiene, ethylene-propylene elastomer, etc.
Despite this scale of production, not much attention has been accorded the exacting requirements for the commodity VCM, except of course by those charged with the responsibility of producing on-spec product VCM. Product VCM, referred to as "finished" VCM, is limited to the following: HCl 0.5 parts per million (ppm) by weight; acetylene (C.sub.2 H.sub.2) 0.2 ppm; caustic (NaOH) 0.3 ppm; butadiene 10 ppm; and, water 100 ppm. The presence of water is undesirable because it causes corrosion in steel shipping containers and storage vessels, generating ferric chloride which seriously interferes with polymerization reactions with the VCM.
As of the present time, VCM produced in a conventional commercial VCM plant is derived from vinyl chloride ("VCl") in the overhead of a VCl distillation column which overhead typically contains from about 50-500 ppm HCl and 10-300 ppm water. This VCl overhead, after it is condensed, is stripped in a stripping column to reduce the level of HCl which is taken overhead. The bottoms from the stripper, still containing in the range from about 1-50 ppm HCl, is scrubbed either by contact with caustic solution, or by upflow percolation through a bed of solid caustic.
For the purpose of clarity, the term "vinyl chloride" (VCl) is used herein when the vinyl chloride is in-process, that is, first formed and subsequently processed in the VCl purification section. The term "vinyl chloride monomer" (VCM) is used when the VCl has been purified, that is finished, so that it meets product VCM specifications.
Those operating a VCM plant utilizing such a process, recognize that corrosion is the overriding problem in the stripping column. This problem is exacerbated because the corrosion process contributes ferric salts to the VCM. Among these salts is ferric chloride which is an impurity which can victimize an otherwise meticulously operated polymerization process because it produces off-spec poly(vinyl chloride). Another impurity which can have the same deleterious effect is butadiene. In copending U.S. patent application Ser. No. 779,337 filed Sept. 23, 1985, there is disclosed a process for minimizing the formation of ferric chloride. Yet it is ferric chloride, in a critically controlled amount, less than 200 ppm which, in combination with chlorine, is so effective in removing butadiene without the FeCl.sub.3 presenting a problem in the subsequent polymerization of the VCM. Yet there is sufficient FeCl.sub.3 present to catalyze the chlorination of BD, and the polymerization of BD, without plugging the quench tower.
About a quarter of a century ago, BD was removed by chlorination at a temperature in the range from -30.degree. C. to 20.degree. C. with an excess of chlorine, in the range from 1 to 5 times the stoichiometrically required amount. The stoichiometric amount is two moles of chlorine for each mole of BD, since BD has two double bonds which can be chlorinated. In this temperature range, the process disclosed in U.S. Pat. No. 3,125,607 to Keating et al was based on the discovery that even a large excess of chlorine did not chlorinate the VCl provided it was anhydrous. But the chlorination took too long even when ferric chloride was used to eliminate moisture. When they used a 2:1 mol ratio (stoich) they found it took 200 min to lower the BD conc from 200 ppm to 20 ppm. The problem with using excess chlorine, was that the chlorine not used in chlorination, remained to hamper the subsequent polymerization of the VCM produced. When a small amount of ferric chloride was added to a VCl-BD (200 ppm) mixture they found no apparent effect. So they used FeCl.sub.3 to pick up moisture; but the FeCl.sub.3 was unavoidably left to contaminate the VCM, it not having been recognized that only a very small amount of left-over FeCl.sub.3 could be tolerated.
FeCl.sub.3 was later found to polymerize BD in VCl, but the polymerization products were adsorbed in a packed column, and were thus removed (see Japanese patent JP No. 49/33165 [74/33165] Sept. '74). This method of removing the BD was contraindicated if the packed column was to operate as the quench tower of a VCl plant.
More than a decade later U.S. Pat. No. 3,876,714 to Coppens taught a process for the addition of from 0.01 to 5% by wt with respect to EDC which was not converted by the pyrolysis, in such a manner as to obtain a liquid practically free of chloroprene (CP), thus ensuring that all BD and CP were chlorinated.
Soon thereafter, U.S. Pat. No. 3,920,761 to Krome disclosed that the addition of from 0.01 to 1% by weight of chlorine (based on the EDC originally used in the feed to the cracking furnace) by itself, was effective to remove BD as well as half the monovinylacetylene (MVA) and two-thirds the CP, all generated at a cracking severity in the range from 70 to 90%, though the temperature range for chlorination with a superstoichiometric amount of chlorine, may be from -30.degree. to +150.degree. C. But no quench tower was used and the reaction was carried out with no limitation on "hold-up" time, as a receiver was used to store cooled effluent from the cracking furnace. Hold-up time is derived by dividing the average instantaneous liquid volume in the quench tower by the rate of flow of liquid EDC (quench stream).
Nor did they note any stringent requirement for anhydrous conditions, or the use of a salt such as FeCl.sub.3 to ensure that such anhydrous conditions be provided. Coppens suggested FeCl.sub.3 as a chlorination catalyst but did not suggest how to deal with the polymers of BD which would also be formed. Presumably, Krome recognized the problems associated with using FeCl.sub.3 but did not realize that less than 200 ppm FeCl.sub.3 would not be injurious to VCM polymerization; nor that it would be an effective catalyst to chlorinate and polymerize BD simultaneously in the operating temperature range of a quench tower.
As is quite evident in either the Coppens or Krome processes, if the cracking severity was 60%, that is, 40% by wt of the EDC was unconverted, then a minimum of 0.4% (4000 ppm) of chlorine would be used. If the BD conc was as high as 200 ppm and it was fully chlorinated, then only 262 ppm chlorine is stoichiometrically required and the remaining excess chlorine must be removed by neutralizing it. This involves the problem of coping with the exces chlorine in the system, not to mention the unnecessary expense.
Moreover, in each of the Coppens and Krome processes, there was no restriction as to hold-up time since a reservoir was used for the chlorination reaction. Further, though Coppens suggested that a chlorination catalyst such as FeCl.sub.3 be added to the reservoir, there is no teaching that the EDC/VCl be essentially anhydrous for the FeCl.sub.3 to provide its catalytic effect, or that there is a strict limit as to the amount of the FeCl.sub.3 which may be carried over into the purified VCM. Particularly noteworthy is the absence of any disclosure as to the polymerization of BD by the FeCl.sub.3, known to have an indiscriminate polymerization effect on diene monomers, or, the formation (or lack thereof) of polybutadiene which would have been easily recognized.